1. Field of the Invention
The present invention relates to a zoom lens system suitable for a single-lens reflex camera.
2. Related Background Art
A zoom lens system has been proposed that is composed of, in order from an object, a first lens group having negative refractive power, a second lens group having positive refractive power, a third lens group having negative refractive power, and a fourth lens group having positive refractive power, wherein the third lens group is composed of a single lens (see, for example, Japanese Patent Application Laid-Open Nos. 2001-183585 and 2004-271937).
However, a zoom lens system disclosed in Japanese Patent Application Laid-Open No. 2001-183585 is a four-lens-group zoom lens having negative-positive-negative-positive power distribution, having a third lens group composed of a single lens element, and having an image circle suitable for a single-lens reflex 35 mm-camera using a 135 film format. Accordingly, when the zoom lens system is made a proportional reduction to obtain a zoom lens system for a single-lens reflex digital camera with a solid-state imaging device of an APS-C size (23.7 mm by 15.6 mm), it causes a problem that the back focal length becomes insufficient.
Moreover, a zoom lens system disclosed in Japanese Patent Application Laid-Open No. 2004-271937 is a five-lens-group zoom lens having negative-positive-negative-positive-positive power distribution, having a third lens group composed of a single lens element, and having an image circle suitable for a digital camera using a so-called 1/1.8″ type CCD imaging device. Accordingly, when the zoom lens system is made a proportional expansion to obtain an image circle for a single-lens reflex digital camera with a solid-state imaging device of an APS-C size, it causes a problem that the zoom lens system becomes too large.
Furthermore, since the zoom lens system disclosed in Japanese Patent Application Laid-Open No. 2004-271937 is not for a single-lens reflex camera, even if the zoom lens system is proportionally expanded, a sufficient back focal length for a single-lens reflex digital camera with a solid-state imaging device of an APS-C size cannot be obtained.